1. Field of the Invention
The present invention relates to a seat height adjustment mechanism provided in a vehicle seat, which is operable to adjustably raise or lower the seat to a desired level. In particular, the invention is directed to the height adjustment mechanism of this kind which is designed to avoid dangerous factor in the case of rear-end collision (i.e. bump from behind).
2. Description of Prior Art
A seat height adjustment mechanism (or the so-called “seat lifter mechanism”) is provided in an automotive or vehicle seat in order to enable a user to adjustably set a height of the seat to his or her desired level. In general, the height adjustment mechanism comprises a pair of forward link members and a pair of backward link members, which are pivotally connected between a seat cushion frame and a seat riser member provided on the side of floor, and also comprises an operating lever for causing vertical movement of those forward and backward link members and a brake unit adapted for limiting downward movement of the link members. For example, the operating lever is operatively connected with the backward link members, so that operation of the operating lever causes vertical displacement of one of the two backward link members, which in turn causes vertical displacement of all other remaining link members. Hence, a user can adjustably raise or lower the seat to a desired level.
In most of the foregoing kinds conventional seat height adjustment mechanisms, the brake unit stated above is provided to only one of the forward and backward link members, which means that a braking effect is only acted on one of the link members from the brake unit, and therefore, all other remaining forward and backward link members are not given any braking effect and thus free to rotate vertically.
Now, if a vehicle with a seat having the foregoing kind of seat height adjustment mechanism is bumped from behind by another vehicle (that is, if a rear-end collision occurs), an excessive great load (a great impact load) is applied from a seat occupant on the seat, under a backward inertia of that seat occupant. In that case, such excessive great load is directly applied to the afore-sated link members without the brake unit, as a result of which, those particular link members are bent and twisted relative to the afore-stated backward link member provided with the brake unit, which may creates the problem that the seat height adjustment mechanism on a whole will be twisted and deformed downwardly and that a serious damage will be given to the seat occupant, or particularly a neck portion of the seat occupant will be damaged.
Recently, to meet safety demands including the above-stated problem, the backward link member of seat height adjustment mechanism is reinforced by increasing the thickness thereof to withstand the afore-said excessive great load. But, such increase of thickness of the link member undesirably results in increase of weight of the seat height adjustment mechanism as well as in increase of costs involved.
As disclosed for example from the Japanese Laid-Open Patent Publication No. 2004-210030, there is known a seat height adjustment mechanism of the type having a lock means which operates to quickly lock a backward link member responsive to a rear-end collision. According to this prior art, the lock means is provided to forward and backward link members which are not provided with a brake unit, and an inwardly toothed hole is formed in the backward link member, and an engagement hook piece is rotatably provided to a seat cushion frame so as to be engaged with or disengaged from the inwardly toothed hole. Also, an actuator arm is connected with the engagement hook piece and movable responsive to a seat belt being stretched by a forward initial displacement of a seat occupant strained by that seat belt, in the case of rear-end collision. Normally, the engagement hook piece is disengaged from the inwardly toothed hole, and therefore, the backward link member is unlocked and free to rotate vertically to permit adjustment in height of the seat. When the rear-end collision occurs, the seat belt is stretched and the actuator arm is quickly moved responsive to such stretching of seat belt to thereby cause the engagement hook piece to rotate toward the inwardly toothed hole of the backward link member and engage the same, so that the backward link member is immediately locked against movement during the rear-end collision. Accordingly, such locking action serves to protect the backward link member (not provided with the brake unit) against deformation by an excessive great load applied to that particular backward link member, thereby avoiding twisting and downward deformation of the seat height mechanism on a whole.
However, the above-described conventional seat height adjustment mechanism is found defective in that, if the load applied from the seat occupant to the seat belt is relatively weak, the seat belt is not sufficiently stretched, and the actuator link will not be moved to cause engagement of the engagement hook piece with the inwardly toothed hole, in which case, the backward link member may not be locked and deformed. Hence, with this conventional mechanism, it is technically difficult to insure to lock the backward link member, and it is likely that the seat height adjustment mechanism may be deformed by actual rear-end collision to give a damage to the seat occupant.